In Lehrer et al. U.S. Pat. Nos. 3,344,300 and 3,398,021 assigned to the assignee of the present invention and respectively entitled "Field Sustained Conductivity Devices with CdS Barrier Layer," and "Method of Making Thin Film Field Sustained Conductivity Device," there are respectively described an electrical field sustained conductivity device and a process for its fabrication. The device consists essentially of a cadmium sulfide layer sandwiched between a pair of electrodes, with one of the electrodes being supported by a transparent substrate. By means of a heat-treating technique described in the patents, a barrier region is created in the cadmium sulfide adjacent to the electrode opposite the one being supported by a transparent substrate, depending upon the particular steps employed in processing the cadmium sulfide.
Devices of the above type have an asymmetrical conductivity so that for a given voltage applied between their electrodes, a much lower current flows through the dielectric when the electrode next to the barrier region is at a lower potential than is the other electrode. This is known as the reverse bias condition of the device and it is in this state that it is ordinarily operated by applying a constant reverse biasing voltage between its electrodes. When the device in its reverse biased condition is exposed to electron bombardment or to illumination, the conductivity of the cadmium sulfide layer is increased and this increased conductivity is retained even after excitation ceases. Thus, current flow is increased in the reverse direction through the cadmium sulfide until the device is restored to its low reverse conductivity state by momentarily interrupting or reversing its applied bias.
A particularly useful application of the device described in the Lehrer patents is the control of an electroluminescent layer for displaying an image. The electroluminescent layer is disposed between one of the electrodes and the cadmium sulfide layer so that conductivity changes sustained in the cadmium sulfide layer change the imposed voltage across the electroluminescent layer and thereby alter its luminescence. Thus, information may be displayed for an extended period of time on the electroluminescent layer by momentarily scanning the device by means of an electron beam modulated with a signal representing the image to be displayed.
An improved method for fabricating a device of the type disclosed in the Lehrer et al. patents is described in Scholl et al., U.S. Pat. No. 3,716,406, also assigned to the present assignee and entitled "Method for Making a Cadmium Sulfide Thin Film Sustained Conductivity Device." The principal feature of the Scholl et al. process lies in the manner of forming the barrier region in the cadmium sulfide layer. In the Lehrer et al. process the cadmium sulfide layer and the electrode adjacent to it are heated in a sulfur-containing atmosphere, with the electrode material being selected to react in such an atmosphere with the cadmium sulfide. In the Scholl et al. method, the electrode adjacent the cadmium sulfide is selected so as not to react with it and a sulfur-containing atmosphere is not used. Instead, a composite film of gold and silicon monoxide is deposited on the cadmium sulfide layer to create the barrier regions. The top electrode is then deposited upon the composite film.
An alternative method disclosed in the Scholl et al. patent includes the deposition of a monolayer of metal particles, such as silver, on the surface of the cadmium sulfide film, followed by a layer of dielectric such as silicon monoxide.
The Scholl process, which represents an improvement over that of Lehrer et al., is believed by applicants to operate through two related phenomena: "storage sites" and "barrier regions". In the case where a composite film such as a mixed co-evaporated layer of gold and silicon monoxide is formed on the cadmium sulfide layer, the particles of gold are believed to create barrier regions, also known as Schottky barriers, on the surface of the cadmium sulfide as well as storage sites in the body of the composite film. It had been previously theorized that the metal particles served to create only the Schottky barriers, and it was believed that the storage sites existed in the cadmium sulfide. The same was also believed to be the phenomena underlying the operation of the device when a layer of silver particles covered by a silicon oxide layer was used.